Hair transplantation procedures typically involve harvesting donor hair grafts and implanting them in a recipient area on a patient. The donor hair grafts are generally harvested from such areas as the back fringe or side areas of a donor's scalp, or other surfaces containing hair. Previously, harvested donor hair grafts were relatively large (3-5 mm). Recent attempts use smaller donor grafts consisting of single follicular units (also referred to as “FUs”), i.e., naturally occurring aggregates of 1-3 (and sometimes 4 or 5) closely spaced hair follicles randomly distributed over a body surface, such as a scalp.
Previous hair transplantation procedures include manual or mechanized procedures featuring some automation. In one manual process, a linear strip of scalp tissue having donor hair grafts is removed with a scalpel down into the fatty subcutaneous tissue. The component follicular units in the strip are then isolated and separated under a microscope. The follicular units are implanted into a recipient area in respective puncture holes made by a needle. Forceps are typically used to grasp and place the follicular units into the needle puncture locations, although other instruments and methods may be used. In an alternative manual process called follicular unit extraction, a hand-held punch or cannula is used to individually extract follicular units from a body surface for subsequent implantation in another location.
Follicular units may be classified based on the number of hair follicles in the unit. For example, an “F1” is a shorthand designation for a follicular unit with a single hair; an “F2” designates a two-hair follicular unit, and so on for F3s, F4s, or follicular units with higher numbers of hair follicles. The distance between a follicular unit and another follicular unit is generally referred to as an interfollicular-unit distance. Specific classes or types of follicular units may be transplanted into specific regions of a scalp. For example, F1s may be implanted along a hairline framing face. Multiple hair follicular units, e.g., F2s, F3s, or greater, are preferably implanted in mid-scalp and crown regions.
A doctor or patient may prefer to harvest only a desired percentage of follicular units and leave some coverage in a donor area. Moreover, a doctor or patient may prefer to evenly distribute the desired percentage amongst remaining follicular units to avoid over-thinning or under-thinning areas of the donor area. However, obtaining an appropriate selection is difficult due to the relatively small size and high number of follicular units, in conjunction with pre-harvested follicular units, image artifacts, occlusion caused by blood or tissue damage, or other discrepancies within the donor area.